Head
unloading
for
increased
hard
disk
drive
shock
tolerance
As
a
result
of
extensive
testing
with
the
computer
lid
both
open
and
closed,
IBM
has
found
that
in
typical
drop
accidents
either
from
the
knee
or
a
desk
to
a
concrete
floor,
the
internal
hard
disk
drive
is
protected
by
the
durable
ThinkPad
shell
so
that
the
received
shock
level
to
the
hard
disk
drive
generally
does
not
exceed
800G
or
800
times
the
force
of
gravity.
Since
800G
is
the
maximum
rating
of
shock
tolerance
of
the
hard
disk
drive
with
the
head
unloaded,
the
hard
disk
drive
can
be
better
protected
during
most
of
these
common
accidents
in
typical
usage
if
the
disk
head
is
unloaded
prior
to
receiving
the
shock
impact.
Head
unloading
can
take
300
to
500ms,
and
the
average
duration
after
detecting
a
free-fall
situation
is
300ms.
Therefore,
it
is
too
late
to
start
head
unloading
after
detecting
the
free-fall
situation.
This
makes
it
necessary
to
concentrate
on
the
behavior
of
the
machine
prior
to
a
drop.
The
initial
behavior
acts
as
a
prediction
of
the
impending
accident.
In
general,
the
duration
of
the
predicting
event
is
much
longer
than
the
actual
free-fall.
This
enables
the
active
protection
system
to
respond
to
the
impending
accident
prior
to
receiving
the
shock.
The
machine
posture
or
system
orientation
is
constantly
checked
by
monitoring
acceleration
and
using
a
prediction
algorithm
to
statistically
analyze
recent
behavior
in
typical
usage.
Head
unloading
is
initiated
by
degrees
of
tilt,
acceleration,
or
shock.
The
degrees
at
which
head
unloading
occurs
are
adjusted
and
fine-tuned
every
time
a
proper
prediction
is
applied
for
the
current
motion.
The
disk
drive
head
remains
unloaded
until
detecting
a
stable
environment
for
approximately
one
to
four
seconds
depending
upon
the
amount
of
excessive
acceleration
at
the
beginning
of
the
current
shock
or
posture
change.
Use
of
Prediction
Algorithm
The
active
protection
system
employs
a
heuristic
learning
algorithm
to
track
system
orientation.
The
Shock
Manager,
a
system
thread
created
by
the
Shockprf.sys
of
a
kernel
mode
device
driver,
analyzes
the
variations
in
acceleration
and
collects
system
orientation
data
into
the
Shock
History
Database.
The
collected
data
is
then
used
to
tune
sensitivity
and
predict
excessive
shocks.
This
tuning
is
important
in
minimizing
the
disk
performance
penalty
caused
by
prediction
failures.
Once
the
Shock
Manager
detects
a
certain
variation
which
may
be
equivalent
to
one
usually
seen
just
before
receiving
an
excessive
shock,
the
Shock
Manager
acts
immediately
to
stop
the
hard
disk
drive.
The
sensitivity
to
predict
a
shock
varies
depending
on
usage
conditions.
In
general,
the
more
stable
the
ThinkPad
computer
stays,
the
more
sensitively
the
Shock
Manager
behaves.
The
Shock
Manager
focuses
its
attention
on
the
current
acceleration
variation
and
the
weighted
average
in
the
recent
past.
The
log
is
used
to
minimize
intrusive
operation
during
normal
usage
of
the
computer.
If
the
hard
disk
drive
head
is
unloaded
after
detecting
a
potentially
harmful
situation,
the
user
can
shorten
the
current
head-unloading
period
up
to
1
second
on
the
condition
that
a
calm
state,
stable
enough
to
remove
the
risk
of
a
drop,
continues
for
at
least
one
second
and
at
the
same
time
mouse
movement
activity
for
more
than
200ms
is
detected
thereby
informing
the
active
protection
system
that
this
is
not
a
drop
situation.
The
design
of
the
active
protection
system
allows
for
certain
shocks
or
vibrations
that
fall
within
a
span
of
accepted
or
normal
motion
and
does
not
repeatedly
stop
the
hard
drive
when
this
level
of
motion
is
detected.
You
can
select
a
checkbox
in
the
properties
window
to
temporarily
disable
hard
drive
protection
while
repetitive
motion
or
vibration
is
detected.
This
feature
is
particularly
useful
when
2
IBM
Active
Protection
System
Whitepaper
